Study on the filling fraction limit of impurities in CoSb3

Complex crystals such as skutterudites have interstitial voids in the lattice that can be filled by various impurity atoms. The filling fraction limit (FFL) for the intrinsic voids in the lattice of CoSb3 is studied by density functional methods. The FFL is shown to be determined not only by the interaction between the impurity and host atoms but also by the formation of secondary phases between the impurity atoms and one of the host atoms. A model is proposed to quantitatively explain the phenomenon. The predicted FFLs for Ca, Sr, Ba, La, Ce, and Yb in CoSb3 are in excellent agreement with reported experimental data. Detailed analysis reveals the existence of a quantitative relationship between the repulsive interaction of impurity atoms and their charge state. A correlation between the FFL of an impurity atom and its charge state and electronegativity is discovered.